Emergent Solvation Behavior in Dual Anion Electrolytes for Zinc Ion Batteries

Abstract

The need for a diverse range of battery materials for an ever-growing energy storage landscape motivates the development of new chemistries with higher capacities and more stable supply chains. Zn-ion batteries (ZIB) are a particularly attractive option due to the abundance and low cost of Zn and the associated cathode materials that can be used to create a full cell. Extensive work on both aqueous and non-aqueous ZIBs has required an increasingly complex set of electrolytes – in many cases incorporating multiple solvents and/or anions to improve overall system performance. It is becoming clear that understanding and, more importantly, controlling the bulk and interfacial Zn2+ solvation structure is of paramount importance to enable improved electrolyte stability and electrode reversibility. In this work we discover the presence of emergent solvation behavior in non-aqueous Zn electrolytes containing multiple anions, where bis(trifluoromethane sulfonyl) imide (TFSI-) anions that are fully dissociated in isolation are shown to form contact ion pairs with Zn2+ when combined with more strongly coordinating halides (X = Cl-, Br-, I-). This effect is demonstrated using both X-ray absorption and Raman spectroscopies and is further supported by first-principles density functional theory calculations, all of which indicate the dominant solvation structure is Zn(TFSI)mono(X), where “mono” indicates a monodentate TFSI- coordination. These emergent solvation complexes are shown to directly impact the electrochemical response, where a significant fraction of Zn2+ complexes in Cl-containing electrolytes are electrochemically inactive at low overpotentials. Additional redox species are activated for metal deposition as the halide association strength weakens (i.e., Cl- > Br- > I-) indicating that, depending on the relative anion-cation coordination strength, apparent improvements in Coulombic efficiency can come at the cost of lower accessible charge/discharge rates. This work suggests a completely new framework for electrolyte design, where anion chemistry can be used to tune both the bulk speciation and the interfacial solvation structure, enabling profound changes to the electrochemical behavior of the system._______________________The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan. http://energy.gov/downloads/doe-public-access-plan